Natural gas is one of mixed gases, containing methane as a constituent of the principal composition thereof. Natural gas has a composition of, for example, 86.73% of methane, 8.86% of ethane, 3.47% of propane, 0.41% of i-butane, 0.52% of n-butane, and 0.01% of nitrogen.
The condition for forming a gas hydrate differs depending on the kind of gas. Generally, gases having larger molecular weight tend to give a hydrate equilibrium condition of lower pressure and higher temperature. Accordingly, gases having larger molecular weight, such as ethane and propane, easily form gas hydrate than methane having smaller molecular weight. In the case of natural gas, therefore, heavier components such as ethane and propane tend to form gas hydrate first, while methane is left behind in the gas phase in large amounts.
Conventionally, in the gas hydrate formation section, a portion of non-reacted gas in the gas hydrate formation system is once carried out therefrom, and then is returned to the system to circulate thereof, which thus increases the formation efficiency of the gas hydrate. The method, however, has a limitation in increasing the efficiency.
According to the above method of the prior art, the gas hydrate formed in the gas hydrate formation section is sent to a cooling section to freeze the gas hydrate by chilling to at or below the freezing point thereof, and the frozen gas hydrate is depressurized in the depressurizing section to a storage pressure, and then the depressurized gas hydrate is sent to the storage section. During the transfer of the depressurized gas hydrate, the non-reacted methane-rich gas which is sent, accompanied with the gas hydrate, from the gas hydrate formation section to the cooling section is further accompanied with the gas hydrate, in the stage of depressurizing to the storage pressure, to enter the storage section. In the prior art, the methane-rich non-reacted gas which is depressurized to the storage pressure near atmospheric pressure is utilized as a fuel outside the system, or is recycled to the raw material system for eliminating loss.
Although gas hydrate is said to have self-retaining properties, the gas hydrate once formed by depressurizing in the pressure-reducing section is decomposed in a part, in some cases. The gas components generated by the decomposition also reach said storage section, and are treated in a similar way.
As a result, in the conventional production method, there were problems that a part of the natural gas as a raw material became a methane-rich gas so as to require different handling, and that the composition of raw material natural gas differs from that of the gas in produced natural gas hydrate.
Since different gas composition gives different calorific value and combustion rate, the gas composition is required to be adjusted so that the gas composition caused by gasification of hydrate and the raw material gas composition become equivalent, which results in increasing the cost.
In order to make the composition of the raw material natural gas and the gas composition of the produced natural gas hydrate equal with each other, there has been proposed a method for producing natural gas hydrate having the steps of: forming a gas hydrate through a reaction between natural gas and water under a low temperature and high pressure condition within the zone where hydrate is formed; freezing the formed gas hydrate by chilling to below the freezing point thereof; and depressurizing the frozen gas hydrate to a storage temperature, wherein the gas components existed after said step of depressurizing is pressurized to return the gas components to said step of forming gas hydrate, (for example, refer to Patent Document 1).
The invention disclosed in Patent Document 1, however, as described above, requires extra pressure-increasing utilities which recycle the methane-rich non-reacted gas conventionally released outside the system to the step of forming gas hydrate. Although the methane-rich non-reacted gas does not go to waste, a long time (for example, 2 to 24 hours) is required until the composition of the raw material natural gas becomes equal to the gas composition of produced natural gas hydrate.
Since the invention disclosed in Patent Document 1 adopts the second formation apparatus at the downstream side of the first formation apparatus, extra utility cost for pressure increase is required. In addition, there are problems of the increase in the building height, and the like.    Patent Document 1: Japanese patent application Kokai publication No. 2005-320454